Anthracnose of Beans

Causal Organism: Colletotrichum lindemuthianum

Symptoms

• Leaves: Small, dark brown to black spots on the veins of the lower leaf surface. Spots may enlarge, becoming angular and leading to defoliation in severe cases.
• Stems and Petioles: Elongated, sunken lesions that are dark brown or black. These lesions can girdle the stem or petiole, leading to wilting or dieback.
• Pods: Circular, sunken lesions with reddish-brown to black margins. Lesions may enlarge, become sunken, and produce pinkish spore masses under humid conditions. Infected pods may rot, and seeds inside can become discolored and shriveled.
• Seeds: Infected seeds show dark brown to black spots or streaks. They are often discolored, shriveled, and have poor germination rates.

Favorable Conditions for Disease Development

• High humidity (above 90%) and moderate temperatures (20–25°C).
• Prolonged leaf wetness due to rain, dew, or overhead irrigation.
• Dense canopy and poor air circulation.

Mode of Spread and Survival

• Primary Source: Infected seeds and crop debris.
• Secondary Spread: Rain splash, wind, and irrigation water.
• The fungus can survive in crop debris for up to two years.

Management

1. Cultural Practices:
   • Use certified, disease-free seeds.
   • Practice crop rotation with non-host crops like cereals for at least two years.
   • Ensure proper field sanitation by removing and destroying crop debris.
   • Avoid overhead irrigation to reduce leaf wetness.
   • Maintain proper plant spacing to improve air circulation.
2. Chemical Control:
   • Seed treatment with Thiram (3 g/kg of seed) or Captan (2 g/kg of seed).
   • Foliar spray with Mancozeb (0.2%) or Chlorothalonil (0.2%) at 10-15 day intervals.
   • Initiate spraying at the onset of disease and continue during favorable weather conditions.
3. Resistant Varieties: Plant resistant or tolerant varieties where available. Continuous use of resistant varieties reduces disease pressure over time.

Bacterial Blight of Beans

Causal Organism: Xanthomonas axonopodis pv. phaseoli

Symptoms

• Leaves: Small, water-soaked spots with yellow halos, which enlarge and merge. Lesions turn brown, becoming angular and restricted by veins. Severe infections lead to blighting and defoliation.
• Stems and Petioles: Water-soaked streaks that enlarge and become brown and sunken. Can cause girdling, leading to wilting and death of affected branches.
• Pods: Water-soaked spots that enlarge and become sunken with reddish-brown margins. Infected pods may crack, leading to seed infection.
• Seeds: Infected seeds are discolored, shriveled, and have reduced germination.

Favorable Conditions for Disease Development

• High humidity and temperatures between 25–30°C.
• Rainy and wet conditions favor bacterial spread.

Mode of Spread and Survival

• Primary Source: Infected seeds and crop residues.
• Secondary Spread: Rain splash, irrigation water, wind, and contaminated tools.
• The bacterium survives in soil and crop debris for up to two years.
• It also spreads through handling of wet plants.

Management

1. Cultural Practices:
   • Use disease-free, certified seeds.
   • Rotate with non-host crops like cereals for at least two years.
   • Avoid overhead irrigation to reduce leaf wetness.
   • Practice good field sanitation by removing and destroying infected debris.
   • Avoid working in wet fields to prevent mechanical transmission.
2. Chemical Control: Spray with Copper oxychloride (0.3%) or Copper hydroxide (0.2%) at 10-15 day intervals. Application of Streptocycline (200 ppm) during early stages is effective.
3. Resistant Varieties: Use resistant or tolerant varieties to minimize yield losses.

Integrated Disease Management (IDM)

• Use a combination of resistant varieties, cultural practices, and chemical control.
• Ensure proper field sanitation, crop rotation, and monitoring.
• Apply chemical sprays as a preventive measure during favorable conditions.